1. Field of the Invention
The present invention relates to an optical module comprising an optical waveguide and a plurality of semiconductor devices integrated on a substrate, more specifically to an optical module which is able to reduce optical noise caused by reflections of leakage lights (or stray lights) in various paths within such a module, thereby reducing crosstalk between semiconductor devices.
2. Description of the Prior Art
Recently, towards construction of an optical subscriber system, necessity for development of low-cost optical modules has been widely recognized. Especially, cost effectivity is important for WDM optical transmitter and receiver modules for multiplexing and demultiplexing 1.3 xcexcm /1.55 xcexcm optical signals and performing bidirectional transmission and reception at 1.3 xcexcm.
With the aim of cost reduction of optical modules, as described in the document (I) below, development is conducted for a hybrid integrated type optical module in which a laser diode (hereinafter in some cases referred simply to as LD), a photodiode (hereinafter in some cases referred to as PD) and the like are disposed directly on a silica optical waveguide substrate. (I) Yamada et al., Preprint of Proceedings for 1996 Spring Conference of the Society of Electronic Communications
FIGS. 27A and 27B are diagrams showing the structure of a prior art optical module, including a perspective view and a sectional diagram showing an important part of the structure of optical waveguide. The optical module shown in FIG. 27A is the one which is described in the above document (I) and has been developed by the inventors.
In the optical module shown in FIG. 27A, a silica optical waveguide 2 is formed on a silicon (hereinafter abbreviated to as Si) substrate 1 provided with irregularities as a substrate, which is referred to a platform. On a Si recess 1a of the platform, an embedded type silica optical waveguide 2 is formed in such a configuration that a core 2a is embedded with a cladding layer 2b of a sufficient thickness. Using the optical waveguide 2, a wavelength multiplexing/demultiplexing circuit (WDM circuit) 101 for multiplexing and demultiplexing 1.3 xcexcm/1.55 xcexcm and a Y-split circuit 102 for 1.3 xcexcm light are formed.
As the wavelength multiplexing/demultiplexing circuit (WDM circuit) 101, a wave multiplexing/demultiplexing function is achieved by a wavelength selection filter 10 inserted in a groove formed in the optical waveguide. Further, on a Si protrusion 1b provided in the vicinity of the end portions of two input/output waveguides of the Y-split circuit 102, a recessed optical device mounting portion 15 is provided which is formed by recessing the optical waveguide substrate 2, and on the thus formed recessed optical device mounting portion 15, a semiconductor chip of LD 30, a semiconductor chip for monitoring PD 32 and a semiconductor chip of a receiver PD 31 are directly mounted.
With this construction, the number of parts constituting the optical module can be substantially reduced. In FIG. 27A, reference numeral 4 indicates an optical fiber connection part, whereas 4a and 4b are optical fibers.
In this optical module, as shown in FIG. 27B, an embedded type optical waveguide 2 is used in which the core 2a is embedded with the cladding layer 2b of a sufficient thickness. Therefore, of the output lights from the LD 30, the components which are not coupled to an optical transmission mode of the optical waveguide 2 are transmitted as leakage lights in the cladding layer 2b, which leak into the optical fiber 4b causing a noise of 1.55 xcexcm port, so that a countermeasure thereto has been required. That is, it has been required to reduce crosstalk lights generated by leakage of 1.3 xcexcm output lights from LD 30 into the optical fiber 4b of 1.55 xcexcm output lights.
As a countermeasure thereto, it is effective to provide a light blocking area which is formed by removing an unnecessary part of the cladding layer 2b while remaining the vicinity of the core 2a (Terui et al., xe2x80x9cOptical Waveguide Circuitxe2x80x9d; Japanese Patent Application Laid-open No. No. 9-5548).
FIG. 28 is a plane diagram showing the structure of an example of an optical module provided with such a light blocking area, wherein the light blocking area 20 is formed by removing an unnecessary area of the cladding layer 2b (which may be referred to just as xe2x80x9ccladdingxe2x80x9d or xe2x80x9ccladding partxe2x80x9d) in front of the recessed optical device mounting portion 15, except the nearby area of the core 2a. With this construction, leakage lights from LD 30 can be prevented from reaching the optical fiber 4b for 1.55 xcexcm output lights. Since the present invention is not directed to a wavelength multiplexing/demultiplexing circuit itself, detailed description thereof is omitted.
The optical module shown in FIG. 28 is provided with a semiconductor chip LD 30 and a semiconductor chip for receiver PD 31 on the same substrate, however, since in an ordinary operation method, the LD 30 and the receiver PD 31 will never be driven simultaneously, turning round of the lights from the LD 30 to the receiver PD 31 is not a problem.
However, when the LD 30 and the receiver PD 31 are to be driven simultaneously, an important problem occurs in the optical module using the embedded type optical waveguide 2. Specifically, because the LD 30 and the receiver PD 31 are disposed in the vicinity of each other on the substrate, the lights outputted from the LD 30 leak into the receiver PD 31, which becomes a noise to the received optical signal. In the ordinary method of use, the LD 30 itself outputs lights of an intensity of +10 to +20 dBm. On the other hand, the receiver PD 31 is required to receive a weak optical signal of less than xe2x88x9230 dBm. Therefore, when receiving such a weak optical signal, the presence of leakage light from the LD 30 has been a critical problem.
From the past, the light leakage path from the LD 30 to the receiver PD 31, as shown by the broken (First Path) line in FIG. 29, of forward and backward output lights from the LD 30, is considered to be mainly a radiation component which is not coupled to the optical transmission mode of the optical waveguide 2 and inputted directly to the receiver PD 31, and the leakage light component has been expected to be prevented, as shown in FIG. 28, by improving the relative positions of the LD 30 and the receiver PD 31 so that the receiver PD 31 is not positioned within the radiation angle of the output lights from the LD 30, thereby preventing the radiation component from the LD 30 from being applied directly to the receiver PD 31.
In addition to the above, the inventors have found that there exist second and third leakage light generation paths as shown by the dotted lines in FIG. 29.
A second leakage light generation position is reflection from a backside wall of the recessed optical device mounting portion 15. That is, some of the backward output lights from the LD 30 are reflected by a backside wall 150 of the recessed optical device mounting portion 15 and an optical waveguide substrate end portion 151, and are incident to the receiver PD 32.
A third leakage light generation path is caused by the light blocking area 20 itself. That is, the output lights from the LD 30 are reflected by a side wall 201 at a side closer to the LD 30 of the light blocking area 20, and incident thereafter to the receiver PD 31. This path can seemingly be prevented by filling the light blocking area 20 with a light absorber, however, in practice, even if it is filled with a light absorber, the third path is inevitably generated so far as there is a refractive index difference between the optical waveguide cladding layer 2b and the absorber.
As described above, the second and third leakage light generation paths are formed by reflection of leakage lights at a refractive index discontinuity portion, and the basic cause thereof is common.
Leakage lights due to the second and third paths become those transmitted to optical devices other than the light emitting devices to generate a noise and, at the same time, are incident again as the leakage lights to the light emitting device itself. As a result, there is a problem in that when the return lights are strong in intensity, it causes a return light noise of the light emitting device itself.
Yet further, in effect, apart from the above described leakage light paths, there is another path in which the leakage lights from these light emitting devices are reflected by a bottom surface or side wall of the optical waveguide substrate itself to enter the light receiving device.
FIGS. 30A and 30B schematically show the state. In this module, the core part 2a and the cladding part 2b of the optical waveguide are formed on a silica glass substrate 10, and the light emitting device 30 and the light receiving device 31 are provided to couple with the core part 2a. However, with such a simple construction, as shown by the arrows in FIG. 30B, stray lights easily reach the light receiving device 31. As a measure for such a problem, heretofore a method to block lights transmitting the above described cladding part, a method of using a wavelength selective filter or the like has been considered.
A construction example shown in FIGS. 31A and 31B is a simplified construction which is applied with a method to block lights transmitting the cladding part (e.g., above-described Japanese Patent Application Laid-open No. 9-5548 xe2x80x9cOptical Waveguide Circuitxe2x80x9d). In this example, as shown in FIG. 31A, a light blocking groove 20 is formed on the surface of the cladding part 2b so that the transmission of stray lights is suppressed by reflection or scattering by the side surface of the groove 20. In this case, the optical module is constructed to be provided with the light emitting device 30 and the light receiving device 31 so that it is connected to an external device by the same output port through the Y-split optical waveguide 2a. 
In general, in such a module, stray lights from the light emitting device 30 not coupled with the optical waveguide 2 enters the light emitting device 31 resulting in the generation of a noise. Geometrical optical paths of stray lights are, for example, as shown by the arrows in FIG. 31B. A greater part of the stray lights are reflected or scattered on the side surface of the groove 20, and the amount of stray lights entering the light emitting device 31 is reduced. As to formation of such a groove, when a silica glass optical waveguide is used as an optical waveguide, since fine processing of silica glass by machining is generally difficult, formation of the groove is performed by a physicochemical method such as plasma etching or the like, different from machining. For this reason, it is very difficult to form a groove of large depth, and a shallow groove is formed on the surface of the substrate. Therefore, stray lights transmitting below the substrate are difficult to be blocked by this groove, and the stray lights of this part reach the light receiving device 31 while repeating reflections.
Furthermore, a construction example shown in FIGS. 32A to 32C is the one that is applied with a method of using a wavelength selective filter (e.g., Inoue et al., Japanese Patent Application No. 9-151825 xe2x80x9cBidirectional WDM Optical Transmission and Reception Modulexe2x80x9d). In this example, the optical module is constructed such that receive light and transmit light have wavelengths xcexin and xcexout differing from each other, these both light waves are respectively transmitted or reflected by the wavelength selective filter 10 and connected through the same port to an external device (FIGS. 32A, 32B). Since the wavelength selective filter 10 has a wavelength selectivity, it can also reflect stray lights from the light emitting device 30 as shown by an arrow in the sectional diagram FIG. 32C.
However, in this method, the wavelength selective filter 10 is inserted in a very narrow groove 12, and, for an insertion of the filter deep into the substrate, it is required to form a groove of a very high aspect ratio, which involves a technical difficulty. Therefore, since a groove is formed with an appropriate depth, this method is not effective to the stray lights transmitting below the substrate as with the above-described example. Moreover, this method cannot be applied to an optical module using the same wavelength.
Still further, it has been found through studies conducted thereafter, noises are generated due to further leakage light paths apart from the reflection by the light blocking groove formed by removing the optical waveguide cladding or at recesses for optical device mounting or the like, and from the stray lights transmitting below the substrate.
For example, in the module of the structure of FIGS. 32A to 32C, another path has been found where a strong scattering of light is generated at the part of the wavelength selective filter 10, and, after repeating multiple scattering, lights reach the light receiving device 31 through a space above the substrate. This path is generated when a structure largely protruding from the substrate is formed as shown in the figures, and had not been recognized as a problem in the past.
That is, in the above structure, considering the fact that optical devices generally having a thickness of about 100 to 200 xcexcm protrude greater than the cladding of optical waveguide generally having a thickness of several tens of xcexcm, leakage lights transmitting through a space over the optical waveguide is investigated, and, as a result, it has been found that the leakage lights have a large influence on the generation of crosstalks.
A first object of the present invention, in order to solve the above-described prior art technical problems in an optical module in which an optical waveguide and optical semiconductor devices are integrated on a substrate, is to provide a technology as a first aspect thereof, which can prevent reflection of the basically horizontal movement of the stray lights from a light emitting device at a refractive index discontinuity part, which reflection is incident thereafter to semiconductor devices.
A second object of the present invention, in order to solve problems with such a prior art optical module, as a second aspect thereof, is to provide a construction for effectively suppressing optical noises due to the lights leaking below the substrate and reflected by the bottom surface or side wall, resulting in a degradation of signals.
A further object of the present invention, in order to solve the problems of leakage lights scattered on the substrate or in the vicinity of the filter and transmitting a space above the substrate, as a third aspect thereof, is to provide a structure of optical module which can efficiently suppress the leakage lights to reduce crosstalk.
An optical module according to the present invention has a silicon substrate, a plurality of optical semiconductor devices integrated on the silicon substrate, and an optical waveguide for performing transmission of optical signals by the optical semiconductor devices, wherein the silicon substrate contains an impurity (dopant) for increasing the number of carriers in the silicon substrate thereby suppressing optical crosstalk between the plurality of optical semiconductor devices.
Further, in particular, to achieve the first object, the optical waveguide comprises a core part for coupling the semiconductor devices with each other on the substrate and a peripheral cladding layer of the core part, or in a construction where each optical fiber is coupled to each semiconductor device, an electrical resistivity of some part or all of the silicon substrate is 0.1 xcexa9cm or less, or a lower part of a light receiving device of the optical semiconductor is made high in resistance and a lower part of a light emitting device of the optical semiconductor is made low in resistance.
To achieve the second object of the present invention, the optical waveguide is an embedded type optical waveguide in which the core part is embedded with the cladding layer, a backside wall of a recess formed in the cladding layer is formed not to be perpendicular to the optical axis of the semiconductor device, and the cladding layer other than the vicinity of the core part is removed to form a further light blocking area in front or rear of the recess in such a manner that the optical waveguide is not divided, wherein the light blocking area formed at the rear of the recess is filled with a black light blocking substance, and the side wall thereof is set obliquely.
Still further, a plurality of recesses are provided, of which between at least those disposed side by side in a longitudinal direction of the optical waveguide, a light blocking area is also formed by removing the cladding layer other than the vicinity of the core of the optical waveguide in such a manner that the optical guide is not divided, the rear side wall is set not to be perpendicular to the optical axis of the semiconductor optical device, or the side wall of the light blocking area is formed not to cross at right angles with the optical axis of the semiconductor optical device.
To attain the third object of the present invention, the optical module has a further filter inserted in a groove formed in the optical waveguide, each of the optical semiconductor devices is locally covered with a transparent resin, the parts protruding upward from the optical waveguide are all coated with a light absorber, and, in this case, either each of them is covered with separate caps or all of them are covered with a single cap.
According to the first aspect of the present invention, in an optical module, all of the leakage light generation paths including generation of leakage lights in the horizontal direction from the light emitting device caused by the presence of a refractive index discontinuity in the optical waveguide can be eliminated, thereby reducing crosstalk optical noises generated due to the leakage lights.
Further, according to the same aspect of the present invention, leakage lights from the light emitting device incident to other optical devices on the same optical waveguide substrate and generate noises can be prevented, and generation of return light noises in the light emitting device can also be prevented.
Still further, according to the second aspect of the present invention, in an optical module in which an optical waveguide and optical semiconductor devices are integrated on a substrate, optical noises due to leakage lights below the substrate degrading the signals can be efficiently suppressed to obtain a high light reception sensitivity, thereby providing an optical module construction of improved functions.
Yet further, according to the third aspect of the present invention, stray lights transmitting above the optical integrated substrate, which have not been taken into consideration in the past, can be efficiently suppressed, thereby enabling an optical module with minimized optical crosstalks. In particular, it is apparent that when the light emitting device and the light receiving device are included in the optical module, the present invention provides an optical module construction which is very effective in achieving an optical module with superior reception characteristics.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.